Our discovery of a photochemical processs for synthesis of fluoroaromatics and fluoroheterocycles has led to the synthesis of a large number of ring-fluorinated analogs of metabolically important imidazoles and phenolic amines. 2-Fluorohistidine, but not the 4-fluoro isomer, is incorporated into newly synthesized animal or bacterial protein, in place of histidine. The resulting enzymes may be structurally intact, but some cannot function catalytically because the pK of the imidazole ring has been greatly depressed. The analog serves as a bacteriostatic agent in completely halting the growth of E. coli in two hours. It is also an antiviral agent, blocking viral multiplication in cell culture by generation of false phosphorylases or viral coat protein. The fluoro analog of 5-aminoimidazole-4-carboxamide riboside is also an antiviral agent, blocking the biosynthesis of both DNA and RNA. A variety of fluoroimidazoles, fluorocatecholamines, and fluoroserotonine are being used as tools to study enzyme and receptor mechanisms, as well as neurotransmission.